1. Field of the Invention
The invention is in the field of cryogenic refrigeration by the method of adiabatic demagnetization.
2. Description of the Prior Art
A widely used technique in cryogenic refrigeration (i.e., refrigeration near the absolute zero of temperature) is the adiabatic demagnetization of a paramagnetic material. One of the earliest materials used for this purpose is the paramagnetic salt, cerium magnesium nitrate. However, at extremely low temperatures, (e.g., less than 0.01.degree. K.) the thermal conductivity of this salt is so low that it is difficult to produce secondary cooling of the material which it is desired to refrigerate. More recently, in order to overcome this problem through the use of metallic conductivity, intermetallic rare earth compounds exhibiting paramagnetism of the Van Vleck type, have been used. The operation of these materials as refrigerants involves the nuclear magnetic entropy and the fact that magnetic fields at the nuclei of Van Vleck paramagnetic ions are much enhanced over the external applied field. In particular, one set of such intermetallic compounds, reaching temperatures between 1.5 and 5 millidegrees Kelvin, are the praseodymium compounds PrTl.sub.3, PrPt.sub.5 and PrCu.sub.6 (Physical Review Letters, 24 (1970) 1181, Physical Review, B10 (1974) 1967 and Journal of Low Temperature Physics, 9 (1972) 267). Interest in the millidegree and submillidegree Kelvin range has been recently sparked by the discovery of unexpected properties of liquid He.sup.3 in this temperature range.